1. Field of Invention
This invention relates in general to a process of manufacturing integrated circuits. More particularly, the present invention relates to a method of making read-only-memory (ROM) components.
2. Description of Related Art
With the progression of semiconductor production techniques, the component density of the ROM device and the masking of the ROM device, in particular, have increased in harmony with the chosen silicon chip size. Moreover, the structure of a basic memory cell unit has also changed from the conventional two-state memory unit structure to the X-cell unit structure, followed by the flat cell unit structure and then to an improved version known as the modified flat cell unit structure.
In general, the commonly used ROM devices employ channel transistors as the basic memory units. Program coding is performed by selectively implanting impurities into designated channel regions so as to adjust their threshold voltages, which sets each conductive channel to an ON or an OFF state.
FIG. 1 is a top view showing a conventional masked ROM device. FIG. 2 is a cross-sectional view taken along line II--II in FIG. 1. Conventional masked ROM devices are formed above a P-doped silicon substrate 100 by first doping N+ ions into source/drain regions 104 which extend in a first direction to form bit lines. Next, gate electrodes 106 are formed, which extend in a second direction perpendicular to the first direction and over a gate oxide layer 102 that covers the silicon substrate 100.
The gate electrodes 106 form word lines. Transistor channels are formed underneath the word lines in areas between two adjacent bit lines. By adjusting the conductivity of the transistor channels, two-state memory units, each having either a "0" state or a "1" state, are formed.
One method of setting a designated transistor channel into an OFF state is by implanting P-type impurities (i.e., performing a coding implant), which results in a higher threshold voltage for those channels. For example, one such transistor 108 is shown in FIGS. 1 and 2.
When the conventional ROM device described above undergoes further component miniaturization, the resistance of the bit lines will increase due to a diminishing line width. As a result, the operational speed of the components may be affected. Additionally, this will entail prominent body effect which will lead to a reduction in component quality.